@Override
 public String addShoppingCart(String accountId, Integer productId, Integer productNum) {
   Account account = accountDAO.find(Account.class, accountId);
   if (account == null) {
     return "账号不存在";
   }
   ProductInfo productInfo = productInfoDAO.find(ProductInfo.class, productId);
   if (productInfo == null) {
     return "商品不存在";
   }
   ShoppingCart shoppingCart = shoppingCartDAO.getShoppingCartByProduct(accountId, productId);
   if (shoppingCart == null) {
     shoppingCart = new ShoppingCart();
     shoppingCart.setProductInfo(productInfo);
     shoppingCart.setCreateTime(new Date());
     shoppingCart.setSellerInfo(productInfo.getSellerInfo());
     shoppingCart.setAccountId(accountId);
     shoppingCart.setProductNum(productNum);
     shoppingCartDAO.add(ShoppingCart.class, shoppingCart);
   } else {
     shoppingCart.setProductNum(shoppingCart.getProductNum() + productNum);
     shoppingCartDAO.update(ShoppingCart.class, shoppingCart);
   }
   return null;
 }
 @Override
 public String modifyShoppingCartNum(String accountId, Integer cartId, Integer modifyNum) {
   ShoppingCart shoppingCart = shoppingCartDAO.find(ShoppingCart.class, cartId);
   if (shoppingCart == null || ObjectUtils.notEqual(accountId, shoppingCart.getAccountId())) {
     return "记录不存在";
   }
   if (shoppingCart.getProductNum() + modifyNum <= 0) {
     return TextConstant.PARAM_ERROR;
   }
   shoppingCart.setProductNum(shoppingCart.getProductNum() + modifyNum);
   shoppingCartDAO.update(ShoppingCart.class, shoppingCart);
   return null;
 }